The goals of our work are to determine the extent to which mRNA populations of adult, immature, regenerating and neoplastic rat liver may differ quantitatively and qualitatively from each other and to identify messenger RNAs (mRNA) and sets of active genes which might be specific for a certain growth pattern. We have established that differences between mRNA populations of normal and regenerating liver are quantitative rather than qualitative and that the transcription of at least two oncogenes increases in parallel with DNA synthesis during liver regeneration. Cells of the normal liver appear to contain most, if not all, of the cellular sequences required for neoplastic transformation; during liver carcinogenesis restriction of genomic expression occurs.